Apprenda, the makers of an on-premise .NET platform-as-a-service, released today a free (as in beer) version of its software called Apprenda Express. The move follows the addition of .NET support to Cloud Foundry last month. Cloud Foundry is a VMware backed (with contributions from companies like Tier 3 and AppFog) free and open source PaaS, that is available both on-premise or in the public cloud. Update: Apprenda Express has actually been available for free since 2010, this is just a routine update.

According to Apprenda, the only restriction in Apprenda Express is a limit of 12 gigs of RAM.

Apprenda is attempting to fill the gap left by Microsoft Azure for an on-premise .NET PaaS. It supports .NET, SQL Server, ASP.NET & Silverlight. In November the company added new features that make it easier to run existing .NET code on Apprenda servers, speeding up migration to the PaaS. This will likely remain its competitive selling point against Cloud Foundry and Microsoft (in the event of an on-premise Azure).

The benefit of running a PaaS on-premise is the ability to quickly provision servers with elastic resource use while keeping them running in a private data center. A company could also deploy a PaaS system to a public infrastructure-as-a-service cloud to gain better control and customization of their PaaS, or to make migrations between providers easier.

IBM is offering its SmartCloud PaaS as both an on-premise and public cloud solution, and AppFog is readying an on-premise version of its custom implementation of Cloud Foundry. The ability of cloud systems to run anywhere – public or private – is quickly becoming table stakes.